The present invention relates to semiconductor memory devices. More particularly, the present invention relates to a high-voltage tolerant interface circuit.
Through the use of a deep-submicron line width in the processing of semiconductor memory devices, a required power supply voltage as high as 3.3V has gradually been decreased to a lower voltage of approximately 2.5V. Most semiconductor memory devices include a plurality of input/output buffers, which operate to buffer data input with such a low power voltage.
Although the power supply voltage level applied to the input/output buffers has decreased, an external high-voltage signal of 5V can also be input to these input/output buffers. As a result, the input/output buffers must have the ability to tolerate a high-voltage of 5V for interfacing with an external 5V-input signal. These 5V-tolerant semiconductor memory devices are commonly used in low-power consumption devices and portable device applications.
However, conventional 5V-tolerant input/output buffers are effective only when an external power voltage is input. In order to protect metal oxide semiconductor (MOS) transistors included in the input/output buffers, it is necessary for the device to have the ability to tolerate voltage levels as high as 5V, even when no external power supply voltage is applied to a semiconductor memory device.
In general, the gates of NMOS and PMOS transistors of the input/output buffers operating with a low power supply voltage are formed having a thin dielectric film, e.g., a thin oxide film, with a low tolerant voltage level. In such a device, if a 5V-signal is applied to the drain or source of the NMOS and PMOS transistors, without the power supply voltage being applied to the gates of the NMOS and PMOS transistor, the gate-source voltage or the gate-drain voltage of the NMOS and PMOS transistors may increase and exceed the tolerable voltage limit of the gates of the NMOS and PMOS transistors. If this happens, the gates of the NMOS and PMOS transistors may b e broken, resulting in improper operation of the device.